1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the semiconductor device.
Note that a semiconductor device in this specification refers to all devices which can function by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic devices are all semiconductor devices.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor including a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (a display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, a semiconductor material using oxide has been attracting attention.
For example, a transistor including an oxide (an oxide semiconductor) containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1.
In a transistor including an oxide semiconductor layer serving as a channel formation region, an oxygen vacancy (an oxygen defect) is generated by release of oxygen from the oxide semiconductor layer, and a carrier is generated due to the oxygen vacancy. It is known that, to solve the above problem, oxygen released from a silicon oxide film containing excessive oxygen is supplied to the oxide semiconductor layer to fill the oxygen vacancy in the oxide semiconductor layer, thereby providing a highly reliable semiconductor device with small change in electric characteristics (Patent Document 2).